This invention generally relates to data management in a data processing and communication station that acts as an interface station between a central or control terminal and a subsystem.
Many large control networks comprise a central control station and a multitude of remote terminals; and, in use, each of the remote terminals is connected to a respective one or more peripheral devices or subsystems, so that each remote terminal acts as an interface station between the central control station and one or more of the peripheral subsystems. Each interface station may be used to transform commands and messages from the central control station into the proper format for transmission to the specific subsystem that is connected to the interface station; and the interface station may be used to monitor the performance of the associated subsystem, and to transform data and messages from the peripheral subsystem into the proper or preferred format for transmission to the central control station.
For example, the central control station may transmit numerous data values to the interface station, where those values may be stored until requested for by the subsystem connected to the interface station. Also, the interface station may regularly monitor and maintain a record of the status of the associated subsystem, and this status record may be transmitted to the central control station at the request of that central station.
Many interface stations that are used in the above-described manner have no or limited control over when changes are made to data at the interface station, or when data is transmitted from the station; and when data and data requests are constantly being transmitted between devices, difficulties may arise as a result of the timing of such transmissions. For instance, the central control station may send data to an interface station before the interface station and the associated subsystem have processed previously sent data. Similarly, the central control station may transmit to an interface station a request for the status or data record of the associated subsystem while the interface station or subsystem is updating that status or data record; and if this happens, the interface station may return to the central control station a mix of the old and new status or data records. Other difficulties may also develop with the integrity of the data in systems in which data requests are frequently being made.
Prior art control networks try in various ways to avoid or to minimize problems of the type described above. For instance, certain operating protocols require a minimum time gap between messages transmitted from a central station to any one remote terminal of the network. This does not guarantee data accuracy, however, because, among other reasons, the minimum time gap may not be sufficient to ensure that data is properly updated between transmissions. Also, commonly both the central control station and each subsystem is provided with a software program to control operation of the respective devices; and often when the subsystem software is being written, the designer does not know of the specific control station, or of the specific control station software, with which the subsystem will eventually be used.